Add stubs for MachineManager testing

It also adds a first test that checks if the setActiveMachine

tests/TestMachineManager.py

@@ -0,0 +1,43 @@
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from UM.Settings.ContainerRegistry import ContainerRegistry
+from cura.Settings.ExtruderManager import ExtruderManager
+from cura.Settings.MachineManager import MachineManager
+
+
+@pytest.fixture()
+def container_registry() -> ContainerRegistry:
+    return MagicMock()
+
+@pytest.fixture()
+def extruder_manager(application, container_registry) -> ExtruderManager:
+    with patch("cura.CuraApplication.CuraApplication.getInstance", MagicMock(return_value=application)):
+        with patch("UM.Settings.ContainerRegistry.ContainerRegistry.getInstance", MagicMock(return_value=container_registry)):
+                manager = ExtruderManager()
+    return manager
+
+@pytest.fixture()
+def machine_manager(application, extruder_manager, container_registry) -> MachineManager:
+    application.getExtruderManager = MagicMock(return_value = extruder_manager)
+    with patch("cura.Settings.CuraContainerRegistry.CuraContainerRegistry.getInstance", MagicMock(return_value=container_registry)):
+        manager = MachineManager(application)
+
+    return manager
+
+
+def test_setActiveMachine(machine_manager):
+    registry = MagicMock()
+
+    mocked_global_stack = MagicMock()
+
+    mocked_global_stack.getId = MagicMock(return_value = "test_machine")
+    registry.findContainerStacks = MagicMock(return_value = [mocked_global_stack])
+    with patch("cura.Settings.CuraContainerRegistry.CuraContainerRegistry.getInstance", MagicMock(return_value=registry)):
+        with patch("UM.Settings.ContainerRegistry.ContainerRegistry.getInstance", MagicMock(return_value=registry)):
+            machine_manager.setActiveMachine("test_machine")
+
+            # Although we mocked the application away, we still want to know if it was notified about the attempted change.
+            machine_manager._application.setGlobalContainerStack.assert_called_with(mocked_global_stack)
+